This invention relates to inductive circuits and, in particular, to circuitry used to drive a deflection yoke for controlling movement of an electron beam in a cathode ray tube (CRT). More specifically, the invention relates to a flyback circuit which enables the deflection yoke to rapidly move the beam from the end of one line trace to the beginning of a subsequent line trace during raster scanning.
A problem inherent in using inductive loads is that rapid changes in current through such a load is met with the characteristic impedance of the load. In order to change the current flow through the load, a substantial increase in potential across the load over a transient time period is necessary in order to effect the change in current rapidly. Some systems using inductive loads must provide power to the load up to a certain maximum amount for the majority of operating time and must provide a substantially increased amount of power to the load for short periods of time. Such circuits include electric motors, and electron beam deflection devices such as video systems which use a magnetic coil deflection yoke.
In certain techniques used to obtain an increase in line resolution at a given frame rate, it is necessary to reduce the time allocated for flyback. In order to accomplish this, flyback boost circuits are used, which increase potential (voltage) to a magnetic deflection yoke in order to cause a rapid change in the yoke's magnetic field. This is because, at higher raster line rates, the time allocated for flyback decreases. If the actual deflection of an electron beam deflected by the yoke lags behind the time allocated for flyback, the result is a distortion on the left side of a screen (assuming a left-to-right horizontal line scan). This would cause the picture to appear stretched or to fold over.
Deflection circuits provide the additional energy needed typically by switching on high voltage across the yoke. High voltage switching must be triggered during flyback. In order to accomplish this, flyback circuits usually require that switching electronics anticipate yoke voltages, in order to be fully conductive when flyback power is to be applied to the yoke. This increases power consumption at the output of the deflection drive circuitry.
Further demands on flyback boost circuits occur when both raster and stroke information are to be displayed. Typically, stroke information is displayed during vertical retrace periods or as a selected alternative to a raster display. Such stroke information involves specific controls of the deflection yoke circuitry and the prior art flyback boost circuits would have to be electrically taken out of the yoke driving amplifiers during such time as stroke writing was used in order to eliminate a degrading effect on the stroke image.
Typical deflection amplifier design utilizes NPN and PNP bipolar power transistors. A problem with such designs is that, when current in the yoke is reversed during flyback, the reversal of current produces a large negative potential (voltage) which will keep NPN emitter voltage lower than the NPN base voltage, and therefore, will keep the NPN transistor on and cause the PNP transistor to saturate. This extends the time required for flyback and can result in a distorted raster display. Additionally, this design results in unnecessary power losses. The use of bipolar power transistors creates further problems because of the large base drive current such a transistor must have before the transistor is fully conductive. The desaturation of the transistor takes time. While not significant in overall power consumption, driving of the base consumes enough current to demand a significant output from a control device.
This invention has, as a principle object, decreased flyback time in order to facilitate increased resolution. It is important that circuitry used to increase flyback time not have a degrading effect on a CRT image produced by stroke writing. It is further important that such a circuit be simple enough in design as to not require separate adjustment. It is accordingly an object of the present invention to provide, to an inductive load, a stable linear current which is proportional to an input driving signal. It is a further object of the invention to enable a deflection amplifier for a cathode ray tube to operate at higher speeds in both a raster scan mode and a stroke writing mode. It is a futher object of the invention to provide a deflection amplifier which consumes a reduced amount of power.